Prerendering: Revisit
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The document discusses various concepts related to web page rendering in Gecko and Chromium, including: 1) It provides an overview of prerendering and how it is implemented in Gecko and Chromium to minimize resource contention and handle dynamic content and cancellations. 2) It describes the docshell architecture in Gecko and how docshells manage browsing contexts and session history. 3) It explains the concepts of inner and outer windows and how windows, browsing contexts, and documents are represented using the Window and WindowProxy interfaces.
![Prerender in Chromium
• Minimizing resource contention.
• Handling dynamic media [video,
audio, plugins, canvas]
• Cancellation of pages on certain
corner cases.
• Minimizing server side effects.
• Mutations to shared local storage
[cookies, sessionStorage, etc.]
Main concerns:
credit: https://www.chromium.org/developers/design-documents/prerender](https://image.slidesharecdn.com/prerendering-160429163022/85/Prerendering-Revisit-6-320.jpg)
![Window
/*sealed*/ interface Window : EventTarget {
// the current browsing context
[Unforgeable] readonly attribute WindowProxy window;
[Unforgeable] readonly attribute Document document;
readonly attribute History history;
// other browsing contexts
[Unforgeable] readonly attribute WindowProxy top;
[Replaceable] readonly attribute WindowProxy parent;
WindowProxy open(optional DOMString url = "about:blank", optional DOMString target = “_blank”,
[TreatNullAs=EmptyString] optional DOMString features = "", optional boolean replace = false);
getter WindowProxy (unsigned long index);
getter object (DOMString name);
// the user agent
readonly attribute Navigator navigator;
};
(Partial) Window IDL definition](https://image.slidesharecdn.com/prerendering-160429163022/85/Prerendering-Revisit-42-320.jpg)
Prerendering: Revisit
- 2. WARNING LONG session. Ask for a break whenever you want. credit: http://www.quickmeme.com/meme/352t2a
- 3. RESOURCE HINTS • dns-prefetch, preconnect, prefetch, prerender
- 4. Resource Hints Concept credit: https://docs.google.com/presentation/d/18zlAdKAxnc51y_kj-6sWLmnjl6TLnaru_WH0LJTjP-o
- 6. Prerender in Chromium • Minimizing resource contention. • Handling dynamic media [video, audio, plugins, canvas] • Cancellation of pages on certain corner cases. • Minimizing server side effects. • Mutations to shared local storage [cookies, sessionStorage, etc.] Main concerns: credit: https://www.chromium.org/developers/design-documents/prerender
- 7. Prerender Cancellation in Chromium • The top-level page is not an HTTP/HTTPS scheme, either on the initial link or during any server-side or client- side redirects. For example, both ftp are canceled. Content scripts are allowed to run on prerendered pages. • window.opener would be non-null when the page is navigated to. • A download is triggered. The download is cancelled before it starts. • A request is issued which is not a GET, HEAD, POST, OPTIONS, or TRACE. • A authentication prompt would appear. • An SSL Client Certificate is requested and requires the user to select a certificate. • A script tries to open a new window. • alert() is called. • window.print() is called. • Any of the resources on the page are flagged by Safe Browsing as malware or phishing. • The fragment on the page does not match the navigated-to location.
- 8. Monitoring Prerender in Chromium
- 9. XPFE • from XUL elements to docshells
- 10. nsWebShellWindow
- 13. browser.xul <tabbrowser/> <browser/> <browser/> <browser/> content.js DocShell XPCOM Module browser.js … browser/base/content/browser.xul browser/base/content/tabbrowser.xml toolkit/content/widgets/browser.xml toolkit/content/widgets/remote-browser.xml browser/base/content/global-scripts.inc browser/base/content/browser.js browser/base/content/content.js major source files Usually referred as “gBrowser”
- 15. browser.xul <tabbrowser/> <remote-browser/> <remote-browser/> <remote-browser/> (content process) browser-child.jsmessage manager browser-child.jsmessage manager message manager browser-child.js content.js browser.js … DocShell XPCOM Module DocShell XPCOM ModulePBrowser.ipdl
- 16. Browser High-level Overview (e10s)
- 17. DOCSHELL • the scary evil thing
- 20. DocLoader & DocShell Tree
- 21. chrome / non-e10s process Chrome or non-e10s Tree nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader do_GetService(“@mozilla.org/docloaderservice;1”) nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell chrome tree owner content tree owner
- 22. chrome / non-e10s process Chrome or non-e10s Tree nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader nsDocLoader do_GetService(“@mozilla.org/docloaderservice;1”) nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell chrome tree owner content tree owner
- 23. e10s content process Content Tree in e10s nsDocLoader nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell nsDocShell do_GetService(“@mozilla.org/docloaderservice;1”) tab A tree owner tab B tree owner
- 26. LoadURI
- 27. LoadURI
- 28. LoadURI
- 29. RENDERING • the very basic concept
- 30. Document Parsing Components credit: http://www-archive.mozilla.org/newlayout/doc/ (slightly outdated)
- 31. Document Rendering Components credit: http://www-archive.mozilla.org/newlayout/doc/
- 32. Rendering Data Flow credit: http://www-archive.mozilla.org/newlayout/doc/
- 33. More on Layout & Rendering Goto http://dbaron.org/talks/
- 34. SESSION HISTORY • browsing context, session history and bfcache
- 35. Browsing context A browsing context is an environment in which Document objects are presented to the user. The docshell is the toplevel object responsible for managing a single browsing context. credit: https://developer.mozilla.org/en-US/docs/Inner_and_outer_windows
- 36. Session History The sequence of Documents in a browsing context is its session history. Session history consists of a flat list of session history entries. Session history entry = URL + state + title + Document + form data + scroll position + …, etc. interface History { readonly attribute long length; readonly attribute any state; void go(optional long delta); void back(); void forward(); void pushState(any data, DOMString title, optional DOMString? url = null); void replaceState(any data, DOMString title, optional DOMString? url = null); };
- 37. Session History
- 41. DOM WINDOW • inner & outer
- 42. Window /*sealed*/ interface Window : EventTarget { // the current browsing context [Unforgeable] readonly attribute WindowProxy window; [Unforgeable] readonly attribute Document document; readonly attribute History history; // other browsing contexts [Unforgeable] readonly attribute WindowProxy top; [Replaceable] readonly attribute WindowProxy parent; WindowProxy open(optional DOMString url = "about:blank", optional DOMString target = “_blank”, [TreatNullAs=EmptyString] optional DOMString features = "", optional boolean replace = false); getter WindowProxy (unsigned long index); getter object (DOMString name); // the user agent readonly attribute Navigator navigator; }; (Partial) Window IDL definition
- 43. WindowProxy credit: http://d.hatena.ne.jp/cou929_la/20110310/1299767973 var w = window.open();
- 44. Split Window In SpiderMonkey, a split object is made up of two JSObjects: an inner object and an outer object. The inner window object is different for each page a browser window visits. It serves as the "globals" object and provides the JSPrincipals for scripts on that page. The outer window object is the object returned by window.open. It represents the window or tab itself and survives as the user navigates in that window or tab. The inner window => HTML5 Window object. The outer window => HTML5 WindowProxy object.
- 46. Nested Windows browsing context credit: https://developer.mozilla.org/en-US/docs/Inner_and_outer_windows
- 47. Nested Windows var w = window.self; browsing context credit: https://developer.mozilla.org/en-US/docs/Inner_and_outer_windows
- 48. Nested Windows var w = window.self; var w = window.parent; var w = window.top; browsing context credit: https://developer.mozilla.org/en-US/docs/Inner_and_outer_windows
- 49. Inner / Outer Window Was Ideal Plan
- 50. Inner / Outer Window Was Ideal PlanNow
- 51. Inner / Outer Window Was Ideal PlanNow
- 52. Inner / Outer Window Was Ideal PlanNow
- 53. WebIDL Bindings 'Window': { 'nativeType': 'nsGlobalWindow', 'binaryNames': { 'postMessage': 'postMessageMoz', }, }, 'WindowProxy': { 'nativeType': 'nsPIDOMWindowOuter', 'headerFile': 'nsPIDOMWindow.h', 'concrete': False }, dom/bindings/Bindings.conf
- 57. Swapping DocShells vs. ContentViewers Swapping DocShells
- 58. Swapping DocShells vs. ContentViewers Swapping Viewers
- 59. Swapping DocShells vs. ContentViewers • Swapping DocShells. • The bfcache issue. • Swapping ContentViewers. • Almost impossible to prerender out-of-process.
- 60. Solutions? • Connects multiple session histories. • With pseudo history entries, or • With mIndexOffset, mExtraLength • How to update SessionStore? • How to manage lifetime of grouped tabs? • Other ways?
- 62. Process Models in Chromium • Process-per-site-instance (*) • Process-per-site • Process-per-tab • Single process credit: https://www.chromium.org/developers/design-documents/process-models http://www.aosabook.org/en/posa/high-performance-networking-in-chrome.html
- 63. General Limitations • Script-connected tabs (unit of related browsing contexts) are always in the same process. http://w3c.github.io/html/single-page.html#groupings-of-browsing-contexts • Chromium only swaps renderer processes for browser-initiated cross-site navigations, such as typing a URL in the location bar or following a bookmark (unless using rel=noreferrer target=_blank). • Subframes are currently rendered in the same process as their parent page (OOPIF is ongoing). • There is a limit to the number of renderer processes that Chromium will create.
- 64. Out-of-Process iframes (OOPIFs) • Process-per-frame, or more preciously unit of related similar- origin browsing context. http://w3c.github.io/html/single-page.html#units-of- related-similar-origin-browsing-contexts • Support more JS-IPC excluding those need access to page content. • On the way to Site Isolation. https://www.chromium.org/developers/design- documents/site-isolation credit: https://www.chromium.org/developers/design-documents/oop-iframes